fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: If690ed2c2e50b86abb1faa4bc1d1238fb688a90f
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -26,6 +26,7 @@ whitelist_externals = bash
                       rm
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t senlin/tests/unit {posargs}
 
 [testenv:debug-py27]
@@ -37,29 +38,35 @@ basepython = python3.5
 commands = oslo_debug_helper -t senlin/tests/unit {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8 senlin
 
 [testenv:genconfig]
+basepython = python3
 envdir = {toxworkdir}/venv
 commands =
   {toxinidir}/tools/gen-config
 
 [testenv:genpolicy]
+basepython = python3
 envdir = {toxworkdir}/venv
 commands =
   {toxinidir}/tools/gen-policy
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source senlin --parallel-mode
 commands = {toxinidir}/tools/cover.sh {posargs}
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -W -b html doc/source doc/build/html
@@ -67,10 +74,12 @@ commands =
 whitelist_externals = rm
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:api-ref]
+basepython = python3
 commands =
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
@@ -89,6 +98,7 @@ local-check-factory = senlin.hacking.checks.factory
 import_exceptions = senlin.common.i18n
 
 [testenv:bandit]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r senlin -x tests -s B101,B104,B110,B310,B311,B506